This invention relates in general to fuel cells, and more particularly to an apparatus and method for removing impurities from a hydrogen fuel supply stream.
Fuel cells are electrochemical cells in which a free energy change resulting from an oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OHxe2x88x92) in alkaline electrolytes. A fuel capable of chemical oxidation is supplied to the anode and ionizes on a suitable catalyst to produce ions and electrons. Gaseous hydrogen is the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density. Similarly, an oxidant is supplied to the fuel cell cathode and is catalytically reduced. The most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as a fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction to be as great as possible. The electrodes must be electronic conductors, and posses the appropriate reactivity to give significant reaction rates. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit. At the cathode, oxygen reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically extracted as vapor. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy and the remainder as heat.
In recent years, nearly all electronic devices have been reduced in size and made lightweight, in particular portable electronic devices. At the same time, energy hungry features such as full color displays, multimedia, large bandwidth data transmission and xe2x80x98always on, always connectedxe2x80x99 have pushed traditional electrolytic battery technology to the limits. Some have sought to replace electrolytic batteries with small fuel cells. The tremendous advantage of fuel cells is the potential ability to provide significantly larger amounts of energy in a small package (as compared to a battery) However, the problem of how to provide the supply of hydrogen fuel to the fuel cell still seeks an elegant and practical solution before widespread consumer acceptance occurs. Even with seven decades behind us since the Hindenberg disaster, consumers remain wary of hydrogen gas, and there is no infrastructure to provide hydrogen to refill exhausted fuel cells.
Some prior art systems use liquid methanol as the source of hydrogen, by catalytically converting or xe2x80x98reformingxe2x80x99 the methanol into hydrogen using miniature reforming devices. U.S. Pat. No. 6,063,515 by Epp and Baumert, assigned to Ballard Power Systems, is one such example, and describes an integrated fuel cell electric power generation system for submarine applications. Liquid methanol is easy to obtain and does not suffer from the same public safety perceptions as does hydrogen gas. One major stumbling block with the reformer approach is that the reforming process does not provide a pure supply of hydrogen gas, but rather the xe2x80x98reformatexe2x80x99 contains trace amounts of oxides of carbon, such as carbon monoxide (CO), which aggressively attack and poison the platinum catalyst in the fuel cell. These impurities in the hydrogen fuel stream result in continuous performance degradation and eventual failure of the fuel cell. To reduce impurities in the reformate requires additional stages in the reforming process and complex, expensive reforming apparatus. One such example is the HOT-SPOT multi-stage reactor by Johnson Matthey, as published in European Fuel Cell News, Vol. 3, No. 2, 1996. These disadvantages negate the use of reformers for small, portable fuel cell systems because of size, weight and cost. It would be an advancement in the art of fuel cell systems to have a small, inexpensive reformer to convert liquid methanol to hydrogen gas coupled to a simple means for reducing contaminants such that performance is not degraded.